Re: [MMUSIC] Query: payload type collision with offer/answer

"Charles Eckel (eckelcu)" <eckelcu@cisco.com> Wed, 09 January 2013 21:01 UTC

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From: "Charles Eckel (eckelcu)" <eckelcu@cisco.com>
To: Harald Alvestrand <harald@alvestrand.no>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] Query: payload type collision with offer/answer
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Date: Wed, 9 Jan 2013 21:01:41 +0000
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Subject: Re: [MMUSIC] Query: payload type collision with offer/answer
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Please see inline.

> -----Original Message-----
> From: mmusic-bounces@ietf.org [mailto:mmusic-bounces@ietf.org] On
> Behalf Of Harald Alvestrand
> Sent: Wednesday, January 09, 2013 11:25 AM
> To: mmusic@ietf.org
> Subject: [MMUSIC] Query: payload type collision with offer/answer
> 
> Hello,
> I have encountered an issue, and I'm not sure it's an issue or not. SDP
> wisdom is sought.
> 
> Suppose two entities exchange the following offer/answer:
> 
> offer from X:
> 
> m=video 97
> a=rtpmap:97 foo
> 
> answer from Y:
> 
> m=video 98
> a=rtpmap:98 foo
> 
> It's clear from RFC 3264 that now X must send codec foo with payload
> type 98, and Y must send codec foo with payload type 97. That's not the
> question.
> 
> But suppose these are the offer and answer:
> 
> offer from X:
> 
> m=video 97 98
> a=rtpmap:97 foo
> a=rtpmap:98 bar
> 
> answer from Y:
> 
> m=video 97 98
> a=rtpmap:97 bar
> a=rtpmap:98 foo
> 
> That is, the payload types collide.
> 
> It would be possible to write code so that X sends foo with 98 and bar
> with 97, while Y sends foo with 97 and bar with 98. But is it conformant
> with the specs to do so?
> 
> And if this has a clear answer - which paragraph of which RFC makes this
> clear?
> (Yes, this discussion is triggered from a real world problem.)

It is recommended to avoid such mismatches when possible, but doing so may not always be possible. The examples you mention are conformant. The following text from section 5.1 addresses this:

   For sendrecv RTP
   streams, the payload type numbers indicate the value of the payload
   type field the offerer expects to receive, and would prefer to send.
   However, for sendonly and sendrecv streams, the answer might indicate
   different payload type numbers for the same codecs, in which case,
   the offerer MUST send with the payload type numbers from the answer.

      Different payload type numbers may be needed in each direction
      because of interoperability concerns with H.323.

That said there are additional requirements placed on payload type usage by some codecs, such as SVC.

Cheers,
Charles

 
>                     Harald
> 
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